Internal Charging Measurements in Medium Earth Orbit Using the SURF Sensor: 2005–2014

The SURF internal charging monitor that measures deposited currents in three shielded aluminum plates is one of the sensors within the Merlin radiation monitoring instrument launched on Giove-A in December 2005 into a 23300-km circular 56° inclination medium earth orbit. The instrument has now completed 8.5 years in orbit despite being originally intended for a two-year operational life. Here, we report on the instrument's health, provide further recent data, and review the overall results in terms of their engineering significance. It is concluded that the instrument remains in good health, with no significant deterioration, and a near continuous data set from December 2005 to April 2014 is now available. The maximum plate currents to date were recorded over the period April 6-8, 2010, during a significant outer electron belt enhancement (also observed by Geostationary Operational Environmental Satellite (GOES)-12 in geostationary orbit), which persisted for several days. From April 7 to April 9, 2010, the current in the most shielded plate (1.0-mm thickness with 1.5-mm shielding) exceeded the widely used 0.1-pA cm
^-2
safety threshold for internal charging (the only days in the mission where this occurred), and on these days, it also exceeded the reasonable worst case current predications given by the DICTAT internal charging tool (the other plate currents remained within DICTAT predictions). The 0.02-pA cm
^-2
safety threshold used in European ECSS charging standards for dielectrics below 25 °C has been exceeded on a far larger number of days; for example, in the most shielded plate, it has been exceeded on 55 days (8% of the total).